MQTT Spy

mqtt-spy is an open source utility intended to help you with monitoring activity on MQTT topics.
It's been designed to deal with high volumes of messages, as well as occasional publications.

mqtt-spy is a JavaFX application, so should work on any operating system with an appropriate version of Java 8 installed.
It's been tried on various Linux distributions (Mint, Ubuntu, RedHat) and Windows 7/8.

mqtt-spy-daemon is a Java-based command line tool that does not require a GUI environment.
Basic functionality works with Java 7, whereas some of the advanced features like scripting require Java 8 to be installed.